A large number of chemical intermediates are produced by catalytic oxidation of an appropriate hydrocarbon feedstock. One of the most important intermediates produced in this manner is vinyl acetate, which is made by oxidation of ethylene and acetic acid in the presence of a platinum group metal catalyst. The process operates in a loop, with modest conversion per pass, so that large amounts of ethylene are recirculated back to the reactor at each pass. The raw gas from the reactor is usually scrubbed with water to remove the product before the gas is recirculated.
Vinyl acetate manufacturing is well known in the art, and is discussed, for example, in U.S. Pat. No. 3,444,189, to Union Oil Co., which describes the synthesis of vinyl acetate by oxidation of ethylene and acetic acid. Other references that teach the general manufacturing process for vinyl acetate include U.S. Pat. No. 3,557,191, to DuPont, which describes a process that uses ethylene to produce acetic acid, which is then reacted with additional ethylene to produce vinyl acetate. U.S. Pat. No. 3,547,983, to Air Reduction Co., Inc., describes the use of ethane instead of ethylene in the production of vinyl acetate.
Many oxidation processes were originally developed using air as the oxygen source, but modem processes frequently operate with a feed of oxygen-enriched air or high-purity oxygen. The use of pure oxygen typically increases yields and reduces or eliminates the need for nitrogen purging from the process loop, since much less inert gas is fed into the loop initially.
Even when oxygen-oxidation is used, however, some purging is necessary. This is because "pure" oxygen is typically slightly less than 100% pure. The most significant other component is argon, with a typical concentration of about 1%. Argon is present in air and, since argon and oxygen have close boiling points, is not well separated in the cryogenic distillation process used to produce oxygen from air.
If argon is not removed, it builds up in the reactor loop, and can adversely affect the reaction dynamics and the flammability of the gas mixture, and/or reduce the life of the catalyst. Therefore, current vinyl acetate production processes normally provide for a small purge stream to be withdrawn from the loop, usually after the vinyl acetate product has been scrubbed out. In addition to argon, the purge gas typically contains unreacted ethylene and oxygen, carbon dioxide, nitrogen, small amounts of methane, ethane and/or propane, and other contaminants such as carbon monoxide, unreacted acetic acid, and water vapor. In prior art processes, this stream is incinerated or used as boiler fuel.
Although the volume of the purge stream is small, its destruction results in the loss, from a typical plant, of about 20 lb of ethylene for every tonne of vinyl acetate produced. At current estimated worldwide annual production of about 4 million tonnes, this represents a feedstock loss of about 40,000 tonnes annually. In a large-scale process of this type, even incremental improvements in efficiency can affect process economics significantly. Therefore, a process that can reduce or eliminate this loss of ethylene feedstock would be valuable to the industry.
Separation of certain gas mixtures by means of selective membranes has been known to be possible for many years, and membrane-based gas separation systems are emerging to challenge conventional separations technology in a number of areas. That membranes have the potential to separate organic vapors from other gases is also known. For example, U.S. Pat. Nos. 4,553,983; 4,857,078; 4,963,165; 4,906,256; 4,994,094; 5,032,148; 5,069,686; 5,127,926; 5,281,255 and 5,501,722 all describe membranes, systems or processes suitable for such separations.
U.S. Pat. No. 4,879,396, to Ozero, discloses a process for removing both carbon dioxide and argon from an ethylene oxide reactor loop by means of an argon-selective membrane, that is, a membrane that preferentially permeates argon and retains ethylene. U.S. Pat. No. 4,904,807, also to Ozero, discloses a process for removing argon from the reactor loop by means of an argon-selective membrane. In both cases, because the membrane is not perfectly selective, a portion of the ethylene is lost inevitably with the argon vent stream.